There are lots of baggage rules, some of which depend on origin, routing, aircraft type, baggage type, season, and contents. Lapses in understanding on the part of the OP can be very costly. There is no good short-cut.
Here is the relevant International contract of carriage. Baggage rules start at clause 97 and go on and on.
http://www.aa.com/i18n/Tariffs/AA1.html